1. Field of the Invention
This invention relates in general to disk drives, and in particular to a method to characterize and limit the effect of disk damage due to head/disk contact in a hard disk drive.
2. Description of Related Art
Moving magnetic storage devices, especially magnetic disk drives, are the storage devices of choice. This is due to their expanded non-volatile memory storage capability combined with a relatively low cost.
Magnetic disk drives are information storage devices which utilize at least one rotatable magnetic media disk having concentric data tracks defined for storing data, a magnetic recording head or transducer for reading data from and/or writing data to the various data tracks, a slider for supporting the transducer in proximity to the data tracks typically in a flying mode above the storage media, a suspension assembly for resiliently supporting the slider and the transducer over the data tracks, and a positioning actuator coupled to the transducer/slider/suspension combination for moving the across the media to the desired data track and maintaining the transducer over the data track center line during a read or a write operation. The transducer is attached to or is formed integrally with the slider which supports the transducer above the data surface of the storage disk by a cushion of air, referred to as an air-bearing, generated by the rotating disk.
The need for higher data density on magnetic disks has imposed a requirement to read and write more data on narrower tracks located on the disk. The achievement of higher data density requires increasingly narrower transducer gaps and increasingly less spacing or clearance, commonly called flying height, between the magnetic transducer and the disk recording surface. In normal operation of the systems designed to reliably record and read data at the higher data density, a magnetic head slider flies on an air bearing about 30 nanometers away from the disk surface. Should the head slider develop abnormal flying characteristics, it can contact the disk surface causing damage to both the head and disk which may lead to damage which results in the loss of stored data.
Various methods are found in the prior art for detecting the high probability of failure as described above. For example, the prior art describes methods and apparatus to measure the flying height of the magnetic heads and providing a warning for taking corrective action when the measurement of flying height indicates an imminent failure condition. IBM's U.S. Pat. No. 4,777,544 granted to Brown et al. describes a harmonic ratio flyheight technique for calculating head/disk spacing. IBM's U.S. Pat. No. 5,410,439 granted to Egbert et al. describes a method and apparatus for measuring head/disk clearances in a disk drive and providing warning of impending failure caused by a head/disk "crash".
Other methods for detecting impending failure include methods of monitoring the readback signal amplitude of heads in a disk drive and comparing with a preset amplitude to monitor disk magnetic coating degradation. These methods assume that loss of readback signal amplitude is associated with disk surface physical damage (for example, abrasive wear damage) caused by the slider contacting the disk and data is either rewritten at a different location or the disk drive is shutdown before a crash occurs.
However, in some situations readback signal degradation may be due to thermal effects caused by head/disk contact or to accumulated effects of small stresses from stray magnetic fields, etc. In these situations, the prior art approaches are not applicable because readback signal degradation is not associated with physical damage of the head/disk interface and the integrity of the disk drive is not at risk.
It therefore can be seen that there is a need for a method and apparatus for monitoring the readback signal quality and for determining whether signal degradation is due to disk magnetic layer damage or whether signal quality can be restored without compromising data integrity.